Creatures fast and slow

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Across
  1. 2. Rhymes with snippet
  2. 6. Aussie boomer
  3. 8. I'm a gnu. How do you do?
  4. 11. Fastest bird on land
  5. 12. Fastest bird in the air
  6. 16. Has a shell but no legs
  7. 17. Indian cat or Leicester footballer?
  8. 18. This cat sounds as if it would not run a fair race
Down
  1. 1. Road crossing with Belisha beacon
  2. 3. South African rugby player
  3. 4. Runs at Southwell, Colwick, Ascot etc
  4. 5. Has a shell and four legs
  5. 7. Is it a dog? Is it a bus?
  6. 9. This bird is fast by name and fast by nature
  7. 10. Fastest fish
  8. 13. British rugby player
  9. 14. The Lazy Runner's deadly sin?
  10. 15. Sounds like a racy 1967 hippie musical